One specialized type of imaging involves the capture of low intensity fluorescence. Briefly, fluorescence is a molecular phenomenon in which a substance absorbs light of a particular wavelength and emits light of a longer wavelength. The absorption of light is referred to as the “excitation”, and the emission of longer wave lights as the “emission”. Both organic and inorganic substances can exhibit fluorescent properties.
Fluorescence imaging is performed by illuminating a sample to excite fluorescence molecules in the sample, and then capturing an image of the sample as it fluoresces using a camera. Such imaging applications present particular challenges to the design of a box or chamber in which the sample is contained during imaging. This is especially true in macroscopic applications where the field-of-view is about 10 cm–30 cm in diameter, as compared to microscopic applications where the field-of-view is less than about 1 cm.
Typically, intensified or cooled charge-coupled device (CCD) cameras are used to detect the fluorescence of low intensity light radiating from the sample. These cameras are generally complex, may require specialized cooling, and are typically fixed to a single location on the top of a specimen chamber. A user places a sample at a predetermined position in the specimen chamber within the field of view for the overhead camera.
Due to this static design, one particular challenge to imaging apparatus design is the diverse lighting needs required during image capture. Fluorescent image capture, of course, involves the sample being illuminated with an in-box illumination source, while the minute amounts of fluoresced from the “excited” sample are detected using a light detector, e.g., a CCD camera.
One problem associated with the capture of overhead images in macroscopic applications is that the relatively large CCD camera is typically centrally located directly over the sample platform which supports the sample. A single illumination source is thus often positioned in the light box at a location off-set from the camera lens, and angularly directed at the sample platform. Thus, for relatively non-planar samples supported atop the platform, substantially uniform illumination is difficult to achieve. Such is also the case when multiple illumination sources are applied which often causes detrimental shadowing, and thus, non-uniform lighting.
Another problem associated with fluorescent imaging in macroscopic applications is that the current imaging apparatus generally employ dichroic mirrors to perform partial filtering functions. Briefly, dichroic mirrors are typically used in fluorescence microscopes to provide an additional amount of separation for the excitation and emission wavelengths. The dichroic mirror is usually mounted at about a 45 degree angle to excitation and emission light. The excitation light is reflected by the dichroic mirror onto the specimen, while the emission light passes through the dichroic mirror, the emission filter, the lens, and is incident on the CCD camera. Dichroic mirrors are commonly used on microscopes because the beam size is very small and so the mirrors are quite compact (usually 1 inch or less in diameter).
For a macroscopic application, as mentioned, the required field-of-view is much larger (i.e., 10 cm–30 cm) than that for a microscopic application (less than about 1 cm). This of course necessitates the use of a much larger lens which in turn renders the use of a dichroic mirror impractical. Due to the size and orientation of such a mirror in the imaging compartment of the imaging box, the footprint of the imaging box is unfeasibly large. In view of the foregoing, an improved illumination assembly for a light box that enables the substantially uniform lighting for fluorescent image capture of the sample would be highly desirable.